In recent years, a technique called RFID for identifying and managing individuals has been studied and developed. In RFID, a medium called an RFID tag includes an integrated circuit (IC) chip such as a large scale integration (LSI) chip or the like, and an antenna. The RFID tag has a function of transmitting and receiving information, such as identification data of an individual or the like, recorded on the IC chip with the antenna via an electromagnetic wave. The information recorded on the IC chip of the RFID tag is read and written by a device called a reader/writer via an electromagnetic wave.
Types of the above described RFID tag are broadly classified into a passive type and an active type depending on a difference of a power supply method.
A passive-type RFID tag has advantages such that the tag can be manufactured with less cost than an active-type RFID tag that internally includes a battery, and does not need to be maintained by taking the lifetime of the battery into account. However, since the passive-type RFID tag does not include a battery that supplies operating power, power demanded to drive the IC chip needs to be transmitted from the reader/writer to the RFID tag simultaneously with a communication. Accordingly, a communicable distance between the reader/writer and the passive-type RFID tag can be decided according to a distance in which power can be transmitted from the reader/writer to the passive-type RFID tag.
As described above, in RFID using the passive-type RFID tag, a frequency band available to a carrier wave is regulated by law or the like since an output of transmitting power is needed for the carrier wave. For example, with regulations laid down by the Radio Act of Japan, a transmission output of an ultra-high frequency (UHF) band such as an 860 to 960 MHz band is higher than that of a high frequency (HF) band such as a 13.56 MHz band. Accordingly, in application fields, such as billing, a prepaid function, and security management, which are desired to limit a communicable distance among application fields of the RFID tag, the HF band can be used because the transmission output may be small. In the meantime, in application fields, such as management of goods, logistics or the like, which are desired to extend a communicable distance, the UHF band can be used because a transmission output is enabled to be increased.
In addition, the RFID tag needs to be designed to fit a size of a target object to which the RFID tag is attached.
For example, if the target object to which the RFID tag is attached is small, also the RFIG tag needs to be downsized according to the size of the target object to which the RFID tag is attached. Especially, since an antenna occupies most of a mounting area of the RFID tag, the antenna needs to be downsized.
However, if the antenna mounted within the RFID tag is downsized, a length of the antenna becomes shorter than a wavelength corresponding to a desired frequency, and a radiation efficiency of the antenna can be possibly decreased. Therefore, a communicable distance between the reader/writer and the RFID tag can be possibly shortened.
Incidentally, there is a conventional technique for configuring an RFID tag antenna by including a power feeding terminal to which an LSI chip for an RFID tag is connected, a loop antenna connected to the power feeding terminal, and a bypass conducting path for bypassing a loop of the loop antenna.
There is also a conventional technique for improving a VSWR characteristic within a needed bandwidth by using a loop antenna having a short-circuit wire for shot-circuiting two points of the loop antenna at a position other than power feeding points provided on the sides of both ends of an antenna conductor that configures the loop antenna in comparison with a multiple-loop antenna where a plurality of loop antennas having different resonant frequencies within the needed bandwidth are connected in parallel.